An organic light emitting device is a semiconductor device for converting electric energy into optical energy. The organic light emitting device includes two opposite electrodes (an anode and a cathode) for applying external power to the organic light emitting device and an organic material layer aligned between the anode and the cathode in order to emit light having a wavelength of the visible ray range when a hole is recombined with an electron. As a forward electric field is applied to the organic light emitting device having the above structure, holes and electrons generated from the anode and cathode are injected into the organic material layer so that the holes are combined with the electrons in the organic material layer, thereby generating excitons. When the excitons have become the ground state, light is generated. Recently, an organic material layer with a multi-layered structure (see, Applied Physics Letters, vol. 51, no. 12, pp. 913-915, 1987) has been suggested in order to allow the holes and electrons generated from the electrodes of the organic light emitting device to be effectively injected or transferred into the organic material layer. If the organic material layer is formed with the multi-layered structure, it is possible to drive the organic light emitting device with significantly reduced operational voltage while improving light emitting efficiency of the organic light emitting device.
In the meantime, various attempts have been made to realize high brightness in organic light emitting devices. For instance, a method of increasing current density obtained by applying an electric field to the organic light emitting device has been suggested. However, since an organic material layer and a thin film structure of the organic light emitting device are weak against heat, higher current density may exert a bad influence upon the organic material layer and the thin film structure of the organic light emitting device, so that the stability of the organic light emitting device may be degraded as the current density increases. For this reason, current studies are focused on organic light emitting devices representing high brightness at low current density.
There are two approaches to obtain high brightness at low current density. The first approach is to use an organic material capable of improving generation efficiency of excitons, which are generated due to recombination of holes and electrons, and/or generation efficiency of photons, which are generated when the excitons have become the ground state. The second approach is to stack at least two organic light emitting device units in series, in which each organic light emitting device unit includes an anode, a cathode and a light emitting section having a light emitting layer capable of emitting light by receiving holes and electrons from the anode and the cathode, respectively. In the following description, a term “stacked organic light emitting device” refers to the structure including at least two organic light emitting device units stacked in series. The light emitting section may include an organic material layer having a multi-layered structure including a hole injection layer, a hole transport layer, a light emitting layer and an electron transport layer, if necessary.
Methods of fabricating the above-mentioned stacked organic light emitting devices have been disclosed in various documents.
For instance, International Publication No. WO95/06400 discloses a stacked organic light emitting device including organic light emitting device units capable of emitting lights having different wavelengths such that light with desired colors can be emitted from the stacked organic light emitting device. According to the above stacked organic light emitting device, each organic light emitting device unit includes two electrodes and a light emitting layer aligned between two electrodes. In addition, the electrodes are connected to the external power source, respectively, in such a manner that the organic light emitting device units can be individually driven.
In addition, International Publication No. WO99/03158 discloses a stacked organic light emitting device including organic light emitting device units capable of emitting lights having the same wavelength such that light with improved brightness can be emitted from the stacked organic light emitting device. The stacked organic light emitting device disclosed in International Publication No. WO99/03158 has a structure similar to that of the stacked organic light emitting device disclosed in International Publication No. WO95/06400, except that the external power source is connected to both ends of the stacked organic light emitting device. That is, external electrodes of the stacked organic light emitting device are connected to the external power source and internal electrodes of the stacked organic light emitting device are disconnected from the external power source.
According to the above-mentioned stacked organic light emitting devices, internal electrodes interposed between the organic light emitting device units include an internal anode in the form of a conductive thin film electrode made from indium-tin-oxide (ITO) or Au having a high work function, and an internal cathode in the form of a metal thin film electrode made from Al (4.28 eV), Ag (4.26 eV), or Ca (2.87 eV). Thus, two-layered internal electrodes, that is, the internal anode and the internal cathode are aligned between the organic light emitting device units of the stacked organic light emitting device while making contact with each other. FIG. 1 shows such a stacked organic light emitting device including two-layered internal electrodes between the organic light emitting device units.
However, according to the stacked organic light emitting device having the structure as shown in FIG. 1, since an ITO-based transparent oxide electrode (internal anode) is formed on a metal thin film (internal cathode), to which electrons are injected, a physical bonding property between the internal anode and the internal cathode is degraded, so the two-layered internal electrodes cannot be effectively formed. In addition, if the internal anode is made from ITO, a sputtering process must be carried out due to the characteristics of the ITO. However, such a sputtering process may increase kinetic energy of atoms (<KeV) as compared with that of an evaporation process (<1 eV). For this reason, if the internal anode is formed through the sputtering process by using ITO, an organic semiconductor thin film already formed may be significantly damaged (see, Journal of Applied Physics, vol. 86, no. 8, pp. 4607-4612, 1999).
In the meantime, European Patent Publication No. 1351558 A1 discloses a stacked organic light emitting device including a single-layered internal electrode made from a single non-conductor thin film having specific resistance above 105 Ωcm without forming two-layered internal electrodes between stacked light emitting sections. The single non-conductor thin film is made from a material capable of simultaneously generating holes and electrons as an electric field is applied to the stacked organic light emitting device in such a manner that the holes and electrons are injected into a hole transport layer and an electron transport layer, respectively. However, the single non-conductor thin film is very expensive and a method of forming the single non-conductor thin film is very difficult.
Accordingly, it is necessary to provide a stacked organic light emitting device, which can be easily fabricated without forming two-layered internal electrodes between stacked light emitting sections.